The present invention relates to electronic circuitry for driving brushless dc motors. More particularly, driving such motors at a constant rotational velocity. Prior art circuitry generally employ a high resolution optical encoder referenced to the motor shaft. From the output of such an encoder, angular information is obtained which is compared to a stable clock signal by a phase comparator which produces an output signal representing rotor phase angle error. The phase error signal is then used to vary the input voltage or current to the motor coils applied by commutation, thus controlling rotational speed. Such configurations require costly encoders along with complex commutation circuits.
The present invention eliminates the need for a high resolution optical encoder and conventional phase lock loop circuits by employing a simple position detector for the commutation needed to start the motor. The circuit of the present invention provides for starting such a motor, for damping initial low frequency instability of rotational speed, and for maintaining substantially constant rotational speed after attaining desired operational speed. Using the signal outputs from Hall effect generators incorporated into the dc motors employed, the present invention also provides damping of unstable rotational speed of the motor shortly after startup and substantially constant rotational speed during normal operation thereafter.
The rotational speed of a dc motor controlled by the drive circuitry of the present invention is locked to a crystal controlled reference signal. Positional information indicating the quadrant in which the rotor is located is used to start the motor. It is also used to correct for momentary load or other operational perturbations which may cause the rotor to unlock from crystal control.